Gas discharge laser with means for removing gas impurities

ABSTRACT

In an excimer laser or a molecular fluorine laser, a heating element ( 12 ) is used which is heated to temperatures in excess of 60° C., in order to remove impurities from the laser gas.

[0001] The invention relates to gas discharge lasers, in particularexcimer lasers or molecular fluorine lasers, which emit laser radiationin the UV range or in the VUV range of the electromagnetic spectrum.

[0002] Such lasers contain suitable gas mixtures in the gas dischargespace. The problem of contamination of the gas mixture has been knownfor a long time. Impurities in the laser gas have negative effects onthe gas lifetime, the laser pulse energy, the stability and otherproperties of the laser. The impurities can both influence the laserprocess itself and, in particular, have negative effects owing tobuild-ups on components of the laser, for example in the form ofdeposits on optical surfaces or on the windows of the gas dischargechamber.

[0003] It is, for example, known in the prior art to remove impuritiesfrom the laser gas by using so-called cold traps.

[0004] DE 198 26 701 A1 addresses the problem of water (H₂O), which isalso known as a detrimental impurity, in the laser gas. The gas mixturesin excimer lasers are usually made up from commercially available gasesand gas mixtures. In this case, commercially available gases aregenerally contaminated with H₂O at a level of 1-2 ppm. This means thatH₂O is re-introduced into the gas discharge space every time the gas ischanged. Even at this concentration, water has a negative effect on theproperties of the laser, in particular the working life. Water can alsoenter the gas discharge space from the outside, or may alternativelybecome desorbed from the surfaces of the walls of the chamber or ofoptical components. The remarks above concerning H₂O likewise apply toother substances as well, for example N₂ and O₂, which are alsoconsidered as detrimental impurities.

[0005] DE 198 26 701 proposes the use of a hygroscopically active mediumin order to remove H₂O from the laser gas.

[0006] It is an object of the invention to provide another effectivemeans for the removal of impurities from laser gas, which removes notonly H₂O but also other impurities.

[0007] The invention is based on the discovery that, when heated toabove 60° C., a heating element has an effect consistent with the objectstated above, and removes impurities such as H₂O, N₂ and O₂. Inparticular, heating elements which consist of a metal or have an atleast partially metallic surface, have been found to be effective.Metals such as Zr, Hf, Ti and W, or alloys which contain at least one ofthese metals, have been found to be particularly suitable.

[0008] The theoretical explanation for the effect of such heatingelements is as follows: at the said temperatures on the metal surface,reactions are induced with H₂O as well as with N₂ and O₂, which lead toa reduction of the said impurities from the laser gas. The heatingelement may be operated both during the laser operation and, especially,when the gas is being changed. It may also be set in operation, i.e.heated to desired setpoint temperatures, at regular intervals(intermittently).

[0009] The heating element according to the invention should not beconfused with a device for regulating the temperature of laser gas in anexcimer laser as known, for example, from the German Utility Model G 9401 808 and likewise from EP 0 783 193 B1 as well. In the latter priorart, the laser gas is warmed to temperatures in the range of from 35° C.to 40° C., in order to improve and stabilise the laser performance. Thepresent invention does not involve this. Rather, the heating elementaccording to the invention causes only local, i.e. spatially limitedheating of the laser gas to temperatures which are sufficient to achievethe effect of purifying the laser gas. The heating-element temperaturesused for this are significantly higher than those temperatures to whichthe laser gas is warmed in the said prior art, in order to achieve theeffect of performance improvement and stabilisation. The effect,according to the invention, of removing gas impurities from the lasergas is further enhanced with an increasing temperature of the heatingelement beyond 60° C. Temperatures above 70° C., particularly preferablyabove 80° C. and, more preferably, above 100° C. are used. Temperaturesof up to 2000° C. or more have shown good results.

[0010] It has been possible to demonstrate an extension of the gaslifetime, and also an improvement in the gas quality, the latter inparticular being shown by the fact that the high voltage for the gasdischarge of the laser could be set to a lower level after using theheating element according to the invention than without heating of theheating element.

[0011] It is possible to use a single heating element, or a plurality ofheating elements may be used at different positions in the gas dischargechamber of the laser. In this case, the overall dimensions of theheating elements, and of their timing controller, are designed in such away that, although impurities are removed owing to local heating,heating of the laser gas as a whole to temperatures which aredetrimental to the laser performance does not take place. These lattertemperatures are encountered in the prior art described above (that isto say, for example, temperatures of around 40° C. in the case of anexcimer laser according to DE-GM G 94 01 808).

[0012] According to a preferred configuration of the invention, theheating element is (or a plurality of heating elements are) arranged ina separate gas space, which can be connected to the actual gas dischargechamber of the laser via lines. In this variant of the invention, it ispossible to provide a separate modular unit, which can be readilyconnected to the gas discharge chamber via the said lines, in order toremove gas impurities in the described way. In this case, valves maypreferably be arranged in the said lines, in order to control the gasexchange between the gas discharge chamber of the laser and the gasspace with the heating element. A pump may also be used, optionally withfilters in which the impurities or undesired reaction products areremoved by the heating element. So that the pump itself does notintroduce any undesired impurities into the laser gas, a dry-runningpump or a diaphragm pump is preferably used. This prevents, inparticular, undesired hydrocarbons from entering the laser gas.

[0013] The invention also concerns a method for removing impurities fromthe laser gas of a gas discharge laser, in particular an excimer laseror a molecular fluorine laser, in which a part of the laser gas isheated to temperatures in excess of 60° C.

[0014] In this method, it is preferable for the part of the laser gas tobe heated to temperatures in excess of 100° C.

[0015] It is furthermore preferable in the method for laser gas flowingpast a heating element (12) to be locally heated.

[0016] Exemplary embodiments of the invention will be described in moredetail below with reference to the drawing, in which:

[0017]FIG. 1 schematically shows an excimer laser with a heating elementin the interior of the gas discharge chamber; and

[0018]FIG. 2 shows an excimer laser with a separate unit which containsa heating element.

[0019]FIG. 1 schematically shows a gas discharge chamber 10 (“lasertube”) of an excimer laser. Details are known to the person skilled inthe art. A heating element 12 made of one of the aforementioned metals(Zr, Hf, Ti, W, or an alloy which contains at least one such metal) isarranged in the interior of the gas discharge chamber 10 of the laser.It is also possible for a plurality of heating elements to be arrangedat suitable positions in the laser, that is to say at positions wherethey do not interfere with the actual laser operation, such as the gasdischarge, or possible optical components and functions. Otherwise,there are no restrictions on the arrangement of the heating element, orthe plurality of heating elements. The heating element may, for example,be configured in the form of a coil, such as an incandescent filament.In proportion to its mass, it should have a surface area which is aslarge as possible, in order to achieve a large effect with a low heatingpower. According to one exemplary embodiment, an incandescent filamentas the heating element, in the form of a coil made of one of theaforementioned metals and with a diameter of approximately 2 to 3 mm,was warmed to temperatures of from 60° C. to 2000° C., and theadvantageous effects described above were achieved. The heating of theheating element 12 is carried out using a controller 14, which suppliesthe heating element with electricity and regulates the current. Forexample, the controller 14 may be used to detect the instantaneoustemperature of the heating element 12 (for example by measuring theinstantaneous resistance) and the current is regulated accordingly inorder to adjust a predetermined setpoint temperature of the heatingelement 12. It is clear that appropriate electrical feeds through thewall of the gas discharge chamber 10, from the controller 14 to theheating element 12, are necessary for this.

[0020]FIG. 2 shows a further embodiment, in which the heating element 12is arranged not directly in the gas discharge chamber 10 of the laser,but rather in a separate gas space 22, which can be selectivelyconnected to the gas discharge chamber via lines 16, 24. Valves 18, 30in the lines 16 and 24, respectively, are used for this. The gas flow inthe lines is marked by arrows. The valve 18 is activated by means of acontroller 20 which, for example, may be controlled by means of thecontrol computer of the laser. A pump 26 in the return line 24 isactuated by a controller 28, and the valve 30 is actuated accordingly bya controller 32.

[0021] The heating element 12 in the gas space 22 is heated by means ofthe controller 14 in a corresponding way to the heating element 12according to FIG. 1. The heating may be carried out, for example, whenthe gas is being changed and/or permanently, or alternativelyintermittently at predetermined time intervals during the laseroperation, in each case with appropriate valve and pump control. To thatend, the valve 18 is opened by means of the controller 20, as is thevalve 30 in the return line 24. The pump 26 is set in operation, so thatgas from the gas discharge chamber 10 enters the gas space 22 via theline 16, and in doing so comes into contact with the surface of theheating element 12. The heating element 12 (represented onlyschematically in FIG. 2) is in this case positioned in the gas space 22in such a way that impurities come into contact with the surface of theheating element with the greatest possible efficiency.

1. Gas discharge laser, in particular excimer laser or molecularfluorine laser, having at least one heating element (12), which can beheated to temperatures in excess of 60° C. and with which the laser gascan be brought into contact, in order to remove impurities from thelaser gas.
 2. Gas discharge laser according to claim 1, characterised inthat the heating element (12) consists of a metal or has a surface madeof a metal.
 3. Gas discharge laser according to claim 2, characterisedin that the metal is Zr, Hf, Ti or W, or an alloy containing at leastone of these metals.
 4. Gas discharge laser according to claim 1,characterised in that the heating element (12) is arranged in a gasspace (22), which can be connected via lines (16, 24) to the gasdischarge chamber (10) of the laser.
 5. Gas discharge laser according toclaim 4, characterised in that at least one valve (18, 30) is arrangedin the lines (16, 24), in order to control gas exchange between the gasdischarge chamber (10) and the gas space (22).
 6. Gas discharge laseraccording to claim 4, characterised by a pump (26) for delivering gas inone of the lines (16, 24).
 7. Gas discharge laser according to claim 4,characterised by one or more filters in the lines (16, 24) or a pump(26).
 8. Gas discharge laser according to claim 4, having a pump (26),which is dry-running or is a diaphragm pump, for delivering gas throughthe lines (16, 24).
 9. Unit for removing gas impurities from the lasergas of a gas discharge laser, in particular an excimer laser or amolecular fluorine laser, having at least one heating element (12),which can be heated to temperatures in excess of 60° C. and with whichthe laser gas can be brought into contact, in order to remove impuritiesfrom the laser gas.
 10. Unit according to claim 9, characterised in thatit can be connected via lines (16, 24) to the gas discharge chamber (10)of the laser in a modular fashion, and can be removed therefrom. 11.Unit according to claim 9 or 10, characterised in that the heatingelement consists of a metal, in particular Zr, Hf, Ti, W or an alloycontaining such a metal, or has at least one surface made of metal, inparticular one of the aforementioned metals.
 12. Method for removingimpurities from the laser gas of a gas discharge laser, in particular anexcimer laser or a molecular fluorine laser, in which a part of thelaser gas is heated to temperatures in excess of 60° C.
 13. Methodaccording to claim 12, characterised in that the part of the laser gasis heated to temperatures in excess of 100° C.
 14. Method according toclaim 12 or 13, characterised in that laser gas flowing past a heatingelement (12) is locally heated.